Downsizing and low-profiling are being ahead in electronic components accompanying the demand for multiple functionalities, high integration, reduced thickness, size and weight in electronic devices. For example, wiring patterns on printed wiring boards are densified, and reductions in weight and thickness of a base material are being designed in suspension substrate for hard-disc.
Further, the demand is increasing for flexible printed wiring boards (FPC boards) that are thin and bendable in printed wiring boards.
Regarding a protective insulating film used in these applications, it is necessary for resist films themselves to be flexible in addition to high resolution and favorable insulating property that have been required. Moreover, a reduction in contaminants generated by the resist film is also desired.
Conventionally, a polyimide film referred to as “coverlay film” has often been used in order to satisfy the above demands. However, this coverlay film requires the use of an adhesive during lamination onto the base material, and producing openings in the joining parts of the elements is also necessary, which has the disadvantage of complicating the processing treatments. Thus, a “solder resist film” which is a photosensitive layer (film) excellent in processability and comparatively inexpensive relative to a polyimide film is used. Examples of a material used for forming this solder resist film include a resist ink composition comprising a reaction product between Novolak type epoxy acrylate and an epoxy compound as disclosed in JP-A S61-243869, a photosensitive resin composition comprising an epoxy acrylate having carboxyl group and an epoxy resin as disclosed in JP-A 2001-264977, and the like.
Although a cured product formed with the above-mentioned composition has superior hardness as a solder resist film, plasticity and flexibility are inferior. When a solder resist film is formed on the surface of a thin printed wiring board, thin suspension substrate for hard-disc, FPC board or the like, warping and torsion of the base material are sometimes generated, and cracking may be occurred in the cured product due to impact on mounting.
When such cracking is occurred, a solder sometimes adheres to unwanted areas during soldering, or absorption of moisture leads to corrosion of a conductor and insulation loss between conductors.
In addition, a cured product formed with a conventional composition has had problems with deterioration of insulation reliability due to hydrolysis under high-temperature and high-moisture conditions.
Moreover, a cured product has sometimes been formed using the above-mentioned composition while the curing temperature is reduced in order to minimize thermal contraction during the curing step. In such cases, however, there have been problems that an unreacted material or a low-molecular-weight impurity remains, outgassing level increases and product yield reduces.
On the other hand, a photocurable and thermosetting resin composition containing an epoxy compound having a defined structure as the primary component has been proposed to lead to a cured product having higher flexibility than conventional compositions that provide solder resist films in JP-A H9-54434.
However, there have been defects that a cured product formed with this composition has insufficient heat resistance and water resistance, and thermal degradation or absorption of moisture sometimes led to corrosion of a conductor and insulation loss between conductors. Furthermore, a photosensitive dry film is often utilized for a solder resist film having higher flexibility in particular. In the production of such photosensitive dry film, there have been problems that cissing occurs during coating onto the base material (support) due to high composition fluidity.
JP-A H8-335767 discloses that a large amount of inorganic filler is used as means for improving heat resistance and improving coatability onto a support. However, if inorganic filler is used in large quantities, there is a case where separation from the cured product sometimes occurs, which leads to product failure due to a particle.
Moreover, since an activating energy beam curable compound having carboxyl group and an epoxy resin readily react, photosensitive resin compositions containing these (JP-A S61-243869 and JP-A 2001-264977) are normally used as a two-part type composition. For this reason, it is necessary to mix the two liquids immediately prior to formation of a resist film, leading to problems with poor workability and storage stability of the composition for forming resist films.